Plasma Growth Hormone and Prolactin Response to FK 33-824, a Synthetic Opioid Agonist, in Broiler Chickens ,

Abstract

A synthetic opioid agonist, FK 33-824 ([D-Ala2, N-Me-Phe4, Met-(O)5-ol]-enkephalin), was administered intramuscularly at levels of 0, 1, 25, or 625 μ g/kg of body weight to 4-wk-old Arbor Acres × Arbor Acres broiler cockerels. All levels of FK 33-824 caused the birds to be sedated, with the highest dose causing deep sedation over the 240-min test period. A significant time by treatment interaction was seen for plasma growth hormone (GH). At 30 min after treatment, a significant increase in GH occurred, but this transitory increase returned to control levels at 60 min. A secondary GH peak was observed in the 240-min samples from cockerels given the 625 μ g/kg dose. Significant treatment and time effects and a time by treatment interaction were seen in the plasma prolactin (PRL) response to FK 33-824. Prolactin was increased at 30 and 60 min after treatment with 1 μ g/kg, whereas higher doses seemed to suppress PRL concentrations. Over the 240-min experimental period, PRL concentrations tended to increase in all treatments except in the treatment with the lowest FK 33-824 dose, but the largest transitory increase was observed at 240 min in the birds given the 625 μ g/kg dose. These results suggest that FK 33-824 affected GH and PRL secretion in chickens in a manner different from other opioid agonists. This difference was probably due to the ability of FK 33-824 to bind to multiple opiate receptors, with the highest affinity for μ receptors and lesser affinity for δ receptors, whereas other opioids have high affinity for δ receptors. Effects mediated by μ receptors may be stimulatory for GH and PRL secretion in chickens, whereas δ receptors may cause inhibitory responses.